FIG. 1 is a block diagram illustrating the general configuration of a linear-type voltage regulator whose output voltage V.sub.out is regulated using a feedback loop. A battery or other unregulated power supply voltage V+ is applied to an input terminal of an output amplifier 10. Output amplifier 10 includes a pass transistor connected between V+ and V.sub.out. A resistor-divided output voltage V.sub.out is fed back into an error amplifier 12, and this feedback voltage is compared to a reference voltage generated by a reference voltage generator 14. The error amplifier 12 generates an error signal which controls the pass transistor in output amplifier 10 to have a conductivity such that the divided V.sub.out voltage matches the reference voltage despite changes in load current.
Output capacitor C is used for both filtering V.sub.out and for frequency compensation to improve the stability of the circuit when transients are created at the V.sub.out terminal. Such transients may be created by varying load conditions. As would be understood by those skilled in the art, the proper selection of the output capacitor C value is dependent upon the impedance of the pass transistor in output amplifier 10.
The impedance of the pass transistor (and thus the output impedance of the regulator) changes as the load current varies. This impedance change can occur even before the feedback circuit reacts to the changed load condition. For example, if the pass transistor were an MOS device having its source coupled to V.sub.out or if the pass transistor were a bipolar transistor having its emitter coupled to V.sub.out, a sudden drop in load resistance would reduce the source or emitter voltage and instantaneously increase the V.sub.GS or V.sub.BE of the pass transistor. This, in turn, decreases the output impedance of the regulator.
Conversely, the output impedance increases when less current is drawn through the load.
This change in output impedance affects the frequency compensation requirements, and the designer must select a value of capacitor C taking this into account. Thus, the capacitor value is selected with worse case conditions in mind.
What is needed is a circuit and method for improving the compensation of a voltage regulator output.
In many types of low dropout voltage regulators, a high voltage depletion mode NMOS device is used as the pass element in output amplifier 10. If it were desired to turn the voltage regulator off, the gate of the depletion mode NMOS device must then be driven to a voltage below its source, which usually means that a negative voltage supply is required to pull the gate below ground. Creating a negative voltage source requires additional complexity and silicon real estate.
What is needed is a circuit and method to turn off a voltage regulator having a depletion mode pass transistor without requiring the creation of a negative voltage supply.
The reference voltage generator 14 in FIG. 1 is typically a band gap reference type, whose characteristics are well known. Band gap voltage generators produce a relatively constant voltage over a range of temperatures by combining a voltage having a positive temperature coefficient with a voltage having a negative temperature coefficient. These voltages are related to the V.sub.BE of bipolar transistors used in the reference voltage generator and are affected by process variations.
The typical band gap reference will have a voltage verses temperature characteristic that peaks at some nominal temperature and decreases in voltage as temperature is increased above or decreased below this nominal temperature. This decrease lowers the reference voltage by a small amount (e.g., up to 5 mV). Part of this decrease is proportional to (kT/q) ln (beta), where beta is the current gain of the bipolar transistors used in the reference voltage generator.
It is important that the reference voltage remain relatively constant throughout a wide range of temperatures and be predictable despite process variations since the ability of the voltage regulator of FIG. 1 to output a constant V.sub.out is directly dependent upon the ability of the reference voltage generator 14 to output a constant reference voltage.
Thus, what is needed is a reference voltage generator whose output is less affected by process dependent beta variations and temperature variations.